The general objective is to determine the basic rules that govern the arrangement and expression of genes in eukaryotic chromosomes, with specific reference to those in Drosophila melanogaster. Using recombinant DNA techniques, we are analyzing five multigene families in D. melanogaster at the molecular level. These are: the 18S and 28S rRNA genes, the histone genes, the heat shock genes at the 87BC locus and two families that code for abundant mRNAs found in many different cell types. These families divide into two classes: tandem and dispersed. The first three belong to the tandem class and are contained in tandemly repeated units confined to single regions in the haploid genome. By contrast, the members of the last two families are dispersed at many different sites throughout the genome. We postulate that whereas members of tandem families are commonly controlled and simultaneously expressed, members of dispersed families are independently controlled, different members being expressed in different cell types. We wish to test this model. We are also attempting to isolate and analyze the genes that respond to the steroid hormone, ecdysone.